1. Technical Field
The present invention relates to oscillation drive devices, physical quantity measurement devices and electronic apparatuses.
2. Related Art
So-called gyroscopes are classified into various types including a rotation type and a vibration type depending on the method of detecting the force acting on an object. Above all, the vibration type gyroscopes are considered to be advantageous for size-reduction and cost-reduction from the viewpoint of their components. Among such vibration type gyroscopes, vibration type gyro-sensors that detect angular velocity acting on an object are known. The vibration type gyro-sensors include a piezoelectric vibration type gyro-sensor that excites a crystal element or a piezoelectric element that is reliable and advantageous in size-reduction. The piezoelectric vibration type gyro-sensor uses a phenomenon in which, when an angular velocity occurs with respect to an object that is vibrating, a Coriolis force is generated in a direction perpendicular to the vibration.
Vibration type gyro-sensors have wide applications, and may be used for, for example, detection of vibration of video cameras, digital cameras and the like, position detection by a global positioning system (GPS) for car navigation systems, posture detection for aircrafts, robots and the like.
In the applications described above, a vibration type gyro-sensor may be operated by a battery. Accordingly, the power consumption of the vibration type gyro-sensor needs to be reduced as much as possible such that the operating life of the battery is to be extended. In this case, it is preferable to stop power supply to the vibration type gyro-sensor while an angular velocity or the like is not detected, and to allow power supply from the battery only when the vibration type gyro-sensor is used. Therefore, after starting the vibration type gyro-sensor, it is necessary to establish its normal operation within a short period of time.
A technology to shorten the startup time of such a vibration type gyro-sensor is described in, for example, Japanese Laid-open Patent Application 2004-286503 (Patent Document 1). Patent Document 1 describes a technology to increase the oscillation amplitude by an amplifier even immediately after activating the sensor, using a structure in which a CR oscillation circuit or a ring oscillator is added within an oscillation loop.
It is noted that, in order to stably detect angular velocities acting on a vibrator, a driver device for the vibration type gyro-sensor needs to vibrate (oscillate) the vibrator stably at a resonance frequency. Also, the vibrator needs to start oscillation and establish its normal operation in a short period of time. Furthermore, the driver device is preferably formed from a small-sized circuit with a low power consumption in order to lower the cost and extend the operating life of the battery.
On the other hand, if the vibrator is made of a crystal with a high Q value and the vibrator is vacuum-sealed inside a package, the driving Q value of the vibrator becomes very high. This causes a problem in that, when the vibrator is excited to generate a driving vibration, it needs a longer time (startup time) for a signal from the vibrator to stabilize.
However, according to the technology provided in Patent Document 1, if the vibrator is to be oscillated with a frequency that is close to the driving frequency of the crystal oscillator, the element area of capacitors and resistances of the CR oscillation circuit becomes large. This brings about a problem which results in a bulkier vibration type gyroscope (vibration type gyro-sensor) and a higher cost. Also, according to the technology provided in Patent Document 1, the vibrator is started initially with a different frequency, and thus is difficult to be brought into a driving frequency of the crystal oscillator with a high Q value. Therefore, if the vibrator is affected by a manufacturing variation or the like, another problem may be caused in that it takes a longer time to establish stable oscillation
Moreover, according to the technology described in Patent Document 1, regardless of whether or not the vibrator is oscillating, energy of the signal from the CR oscillation circuit is injected in the vibrator. In this case, the energy with a predetermined fixed frequency is applied regardless of the resonance frequency of the vibrator, such that the signal of the CR oscillation circuit would become hindrance to the stationary oscillation of the vibrator as the vibrator approaches to its stationary oscillation. Accordingly, in order to shorten the startup time up to the stationary oscillation of the vibrator, the energy needs to be injected into the oscillation loop in a manner not to be substantially apart from the stationary oscillation condition of the oscillation loop including the vibrator to the extent to cause hindrance to oscillation of the vibrator.
Furthermore, if excitation of the driving vibration of the vibrator is performed simply by a comparator, and the energy of the signal from the CR oscillator circuit is injected at the input side of the comparator, the comparator generates an output signal with high frequency components. This causes a problem in that the timing of the energy given by an equivalent circuit constant of the vibrator would become substantially shifted away from the stationary oscillation condition, which may cause a startup failure.